Garage and root cellar insulation
I received a call from my mechanical engineer today asking me how I planned to insulate the garage and root cellar ceilings. Both rooms are in the basement and above those ceilings are first floor living spaces. I told him my plan is to dense pack them with cellulose and probably add an inch of Polyiso covered with a layer of gypsum to break the thermal bridge and satisfy fire code. I was planning on insulating the rim joists with a couple of inches of ccSF on the inside, the only area in the house with the nasty stuff, mineral wool on the outside, and then dense pack would fill the bays.
Anyone who has ever lived in a house with a garage below a living space knows how problematic this detail can be. I once lived in a house with a bedroom above the garage and it was always cold in the winter. In our attempt to fix the issue, we discovered a few issues with the way the way the “builder” insulated the space. First is there was plumbing going down the middle of one of the joist bays and they just stuffed R38 batts between the PEX and the gypsum, so in a 12′ deep joist bay, the R38 covered only the bottom half of the bay. If this was only in the middle of the garage, maybe not so bad, but there was a tub next to an exterior wall and so half the rim joist was uninsulated and well as half the joist bay. Next there were two bathroom exhaust vents going through the bays so a nice 4″ aluminum flex duct providing a thermal bridge the entire length of 2 joist bays, with the same compressed R38 issue. The obvious fault was the thermal bridge of the joists themselves. Finally, the R38 when fitted in the I-joists created a gap around both the top and bottom cord. Since the rim joists were not separately insulated, we had an uninsulated section of the floor above open to an uninsulated section of the rim joist that provided a path for warm air to escape.
I think the detail I am contemplating for the new house overcomes those issues because:
1) No mechanicals in those ceiling sections to mess up my insulation, 2) Completely sealed and insulated rim joists and 3) thermal break provided by the Polyiso, There is the concern that DP will settle over time, but if the space is properly insulated below and in the rim joists, then that air gap should not really be issue, right? Any other thoughts on how to better insulate these spaces?
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